Primary teeth are often treated as temporary and therefore unimportant. Their role in positioning the permanent teeth is the main argument against that view.
The primary teeth are wider than their successors
The two primary molars in each quadrant occupy more combined width than the two premolars that eventually replace them.
That surplus, sometimes called leeway space, is available for the permanent molars to shift forward slightly and for crowding at the front to resolve.
How the surplus is used is determined by which teeth are present while the permanent teeth are erupting, which is why early loss changes the outcome.
Neighbouring teeth drift into gaps
Teeth are not fixed in position. They drift toward any adjacent space, tipping and rotating as they move rather than travelling upright.
When a primary molar is lost early, the tooth behind it migrates forward and takes up part of the space intended for the permanent successor.
The permanent tooth then erupts into insufficient room, emerging out of the arch or becoming blocked from erupting at all.
Roots guide the permanent tooth's path
As a permanent tooth develops beneath a primary one, it presses against the primary root and prompts specialised cells to resorb it.
The remaining root acts as a channel, directing the erupting tooth toward the correct position in the arch.
Losing that guide before the permanent tooth is ready removes the reference, and the tooth may erupt in an unintended direction.
Space maintainers substitute for the missing tooth
Where a primary tooth is lost well before its natural exfoliation, a small appliance can be fitted to hold the adjacent teeth apart.
These devices do not move teeth or create space. They preserve the dimension that already exists until the permanent tooth arrives.
Whether one is needed depends on which tooth was lost, the child's age and how far the successor has developed, all of which are read from radiographs.
The reasoning behind treating primary teeth
Decay in a primary tooth can progress quickly because the enamel and dentine layers are thinner than in permanent teeth, leaving less distance to the nerve.
Infection at the root of a primary tooth also sits directly above the developing permanent tooth, which is why it is not simply left to fall out.
Decisions about whether to restore or remove a primary tooth are made by a dentist weighing its remaining lifespan against the position it holds.